VLBA Implementation of the ROACH Digital Backend

Jonathan Romneyon behalf of the VLBA Upgrade development team

NRAO / Socorro

First International VLBI Technology WorkshopHaystack Observatory

2012 October 23

22012 / 10 / 23 – jdr 1st Int'l VLBI Technology Workshop

Context

RDBE: Central Element of VLBA Sensitivity UpgradeOriginal goal (April 2007): “4 Gbps by 2011”.Almost made the date

Routine scientific observations started 2012 / 2 / 19.BUT only at 2 Gb/s

Instrumentation was designed for 4 Gb/s.Expansion 2 => 4 Gbps “only requires additional funding”.

Developed in Various CollaborationsNRAO with KAT and CASPER to develop Roach (1) board.NRAO with Haystack on firmware for Xilinx Virtex-5 FPGA.First test 2010 / 7 / 8: “What could have gone wrong?” (AEN).Operational for wideband continuum observations: 2012 / 2 / 19.

1st Int'l VLBI Technology Workshop 32012 / 10 / 23 – jdr

Dual-RDBE Configuration

Planned from the beginning ...to support VLBA's four-IF modes and 4-Gb/s data rate.

Additional, more recent and essential requirementEssential for 8-channel spectroscopic observations with DDC.

Currently using only single RDBEInstallation of second set recently completed.

Software-based network switches bought from XCubeSequence two packet streams to record on one Mark 5C unit.

1st Int'l VLBI Technology Workshop 42012 / 10 / 23 – jdr

PFB (polyphase filterbank) FPGA Personality

Developed by HaystackHas become our workhorse for ongoing scientific programs

Used in 108 observations since 2012 / 2 / 19, through 2012 / 9 / 30, a total of 900 array-hours, about 17% of full time, 33% of obs time.

Fixed configuration16 x 32-MHz sub-bands.Selectable among total of 30 at 32-MHz steps in both IF inputs.Samples in second Nyquist zone, at 1024-MHz sample clock.Requantized after filtering to 2-bit samples for recording.

Significantly inconvenient in VLBA applicationSub-bands fixed in IF frequency mismatched to VLBA's (current) inflexible

LO generator (500*n ± 100 MHz).

Still using PFBG_1.4; have not upgraded (yet) to 1.4.1Fixing fourth-channel phase flip not worth down time to validate.

1st Int'l VLBI Technology Workshop 52012 / 10 / 23 – jdr

DDC (digital downconverter) FPGA

PersonalityDeveloped by NRAO — Still in commissioningBasic specifications

1, 2, or 4 sub-bands, selectable between two IF inputs.Bandwidths 128 / 64 / 32 / ... / 1 MHz.

DiFX spectral zoom may obviate need for narrowest bandwidths.Sideband selectable relative to band edge.Sub-bands may not cross IF zone boundaries at 640 and 896 MHz.

Fine band-edge tuning possible, in ~59-mHz steps.But prefer 15.625 kHz for integral-Hz operation ...Or 250 kHz for compatibility with 10-kHz “legacy” systems.

Requantized after filtering to 2-bit samples, more possible eventually.

Additional featuresAdvanced requantizer level-setting algorithm.Multi-thread VDIF output formatter, required for use with switch.

1st Int'l VLBI Technology Workshop 62012 / 10 / 23 – jdr

Control Software forVLBA RDBE

Extension of EVLA Control SystemFacilitated rapid implementation.Provided well-understood monitoring/logging system.Integrated system; also supports Mark5C (via DRS) and 4x4 IF

matrix switch.

BUT Not PortableUse requires availability of many parts of EVLA system,

including monitor database.Thus, NRAO not participating in compatibility testing this week.

2012 / 10 / 23 – jdr 1st Int'l VLBI Technology Workshop 7

Aside on Bandpass Shapes

Bandpass RationalizationBandpass shapes seen in RDBE data always appear so ugly

and variable.

1st Int'l VLBI Technology Workshop 8

2012 / 10 / 23 – jdr

IF 1 IF 4

2012 / 10 / 23 – jdr 1st Int'l VLBI Technology Workshop 9

Aside on Bandpass Shapes

Bandpass RationalizationBandpass shapes seen in RDBE data always appear so ugly

and variable.We expect digital filters to have a deterministic shape.Usual hand-waving explanation: “The shape of the underlying

instrumental bands, at RF and IF, is showing through.”Sounds reasonable, but is it true?An early test with the PFB personality confirmed this concept. Eight contiguous dual-polar sub-bands were set at three

different 32-MHz offsets.Each of six RF ranges appeared in three different sub-bands,

i.e., AIPS “IF”s.

1st Int'l VLBI Technology Workshop 10

2012 / 10 / 23 – jdr

8256 MHz 8288 MHz

1st Int'l VLBI Technology Workshop 11

2012 / 10 / 23 – jdr

8320 MHz 8352 MHz

1st Int'l VLBI Technology Workshop 12

2012 / 10 / 23 – jdr

8384 MHz 8416 MHz

1st Int'l VLBI Technology Workshop 132012 / 10 / 23 – jdr

Thank you

1st Int'l VLBI Technology Workshop 142012 / 10 / 23 – jdr

“Say It Ain’t So, Babe”

Disheartening recommendation by NSF Portfolio Reviewthat NSF “divest” the VLBA

We don’t know yet exactly how NSF will interpret “divest”.

We continue to seek new funding partners and to reduce operational costs.

We are investigating whether e-VLBI could be more cost effective than disk transport.

The overall capabilities of the VLBA are likely to be reduced.

Some VLBA antennas may be closed.

1st Int'l VLBI Technology Workshop 152012 / 10 / 23 – jdr

Los Alamos

1st Int'l VLBI Technology Workshop 162012 / 10 / 23 – jdr

North Liberty

1st Int'l VLBI Technology Workshop 172012 / 10 / 23 – jdr

Hancock